How can I tell if a caster wheel is made of rubber or polyurethane?

I have a lot of wheels that I want to sell, but do not know if the wheels of rubber or polyurethane. Is there any evidence or touch the main visual difference / between the two materials that will help me determine what are those facts?

Rubber strain crystallizes. One of you put wheels under intense pressure, for example a press, then, if it crystallizes, which means they shatter, then you have rubber. Both materials are polymers and this is the only way I can think of giving the information I have. Hopefully, a expert will come and handle this.

8 PC 3" RUBBER CASTER WHEELS 4 FIXED AND 4 SWIVEL W-8 $14.99

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3 Black Rubber Caster with Brake - Set of 4 by InterMetro $55.96 Let your furniture go mobile!These Black Rubber Casters with Brake by InterMetro let you convert your old shelf rack into a storage trolley. These casters makes life so much simpler; you can now re-arrange your furniture to your heart's content. The casters are quite easy to install; simply screw them into the base of the four posts of your furniture, and you're set. Each caster comes equipped wit...